1. Field of the Invention
The disclosed invention relates to filter systems in general and, in particular, to a self-tuning filter for data acquisition.
2. Description of the Background
It is known that the acquisition of data at high sampling rates, typically greater than 5000 samples per second, is a difficult process and that the sampled data is very prone to sampling error due to noisy signals. Such noise is an inherent problem in many applications such as in internal combustion engines. That noise can lead to what is called “aliasing” which is caused by interference of the noise frequency with the sampling frequency. An aliased signal is a false signal that results from a sampling rate that is less frequent than desirable. Where a sensed condition changes very rapidly, it may be difficult or impossible to sense the condition at a frequency that eliminates aliasing.
To resolve the aliasing problem in such circumstances, it is common to filter the signal by applying a low-pass filter to the signal between the sensing device and the data acquisition device. Such a low-pass filter may be setup as an “anti-aliasing” filter by setting a filter corner frequency of the anti-aliasing filter at half of the sampling frequency. Such filtering, however, introduces additional problems as the filtering process delays the signal causing a timing mismatch. To resolve the timing mismatch, it is preferable to have a programmable filter controlled by the data acquisition unit so that the data acquisition unit can control the level of filtering being applied and compensate for the timing mismatch associated with the corner frequency set at the anti-aliasing filter.
The approach of using a programmable anti-aliasing filter is complicated, however, if the sampling frequency changes with time, such as in the situation of degree-of-rotation based sampling of an engine operating characteristic during transient operation. In such a case, the filter frequency needs to change to match the changing sampling frequency to maintain measurement accuracy.
Thus, there is a need for an adjustable low-pass filter system that can follow a changing sampling frequency.
There is a further need for a digital frequency multiplier that has improved operating characteristics over a phase-lock loop.
Accordingly, the present invention provides solutions to the shortcomings of prior input filtering devices and frequency multiplying devices. Those of ordinary skill in the art will readily appreciate, therefore, that those and other details, features, and advantages will become further apparent in the following detailed description of the preferred embodiments.